1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for manufacturing the liquid crystal display device and a CF (Color Filter) subtrate and more particularly to the liquid crystal display device in which a columnar spacer used to secure a cell gap between the CF substrate on which a colored layer is formed and a TFT (Thin Film Transistor) substrate on which the TFTs are formed, and to the method for manufacturing the liquid crystal display device and the CF substrate.
2. Description of the Related Art
The liquid crystal display device is widely used as a display device of various kinds of information devices or a like. In general the liquid crystal display device is so configured that a liquid crystal is put into a cell gap, in a hermetically sealed manner, between a CF substrate on which a colored layer having a plurality of one set of pixels made up of three kinds of unit pixels including a unit pixel for a red (R) color, a unit pixel for a green (G) color, and a unit pixel for a blue (B) color is formed; and a TFT substrate on which TFTs are adapted to operate as switching elements is formed on a face being opposite to the colored layer. Such the liquid crystal display device is roughly classified into two types, one being a TN (Twisted Nematic)—type liquid crystal display device and another being an IPS (In-Plane Switching)—type liquid crystal display device, depending on its display method.
FIG. 17 is a cross-sectional view schematically showing configurations of a conventional TN-type liquid crystal display device. As shown in FIG. 17, a liquid crystal 71 (liquid crystal molecule) is put into a cell gap 70, in a hermetically sealed manner between, between a TFT substrate 51 and a CF substrate 61. Moreover, the TFT substrate 51 includes a first transparent substrate 52 made up of glass or a like, a first polarizer 53 formed on a rear of the first transparent substrate 52, a pixel electrode 54 formed on a surface of the first transparent substrate 52, an interlayer dielectric 55 formed in a manner that it covers the pixel electrode 54, a drain wiring 56 formed on the interlayer dielectric 55, a passivation film 57 formed in a manner that it covers the drain wiring 56, and a first oriented film 58 formed on the passivation film 57.
The CF substrate 61 includes a second transparent substrate 62 made up of glass or a like, a second polarizer 63 formed on a rear of the second transparent substrate 62, a common electrode 64 formed on a surface of the second transparent substrate 62 and a BM (Black Matrix) layer 65 formed on the surface of the second transparent substrate 62, a colored layer 66 covering the common electrode 64 and the BM layer 65, an OC (Over Coat) layer 67 covering the BM layer 65 and the colored layer 66, and a second oriented film 68 formed on the OC layer 67.
In the TN-type liquid crystal display device described above, by applying a driving voltage between the pixel electrode 54 of the TFT substrate 51 and the common electrode 64 of the CF substrate 61, an electric field in a longitudinal direction relative to both the TFT substrate 51 and the CF substrate 61, is produced, as indicated by arrows 72.
FIG. 18 is a cross-sectional view schematically showing configurations of a conventional IPS-type liquid crystal display device. The configurations of the IPS-type liquid crystal display device shown in FIG. 18 differ from those in the TN-type liquid crystal display device described above in that the pixel electrode 54 and the common electrode 64 are formed on the first transparent substrate 52 in the TFT substrate 51 so that the pixel electrode 54 and the common electrode 64 are insulated from each other with the interlayer dielectric 55 being interposed between the pixel electrode 54 and the common electrode 64.
In such the IPS-type liquid crystal display device as described above, by applying a driving voltage between the pixel electrode 54 and the common electrode 64 formed on the TFT substrate 51, an electric field in a horizontal direction relative to the TFT substrate 51, is produced, as indicated by arrows 73. By configuring above, in the case of the IPS-type liquid crystal display device, a direction of the liquid crystal molecule 71 along the surface of the TFT substrate 51 is determined, which can provide a wider viewing angle compared with the case of the TN-type liquid crystal display device. Therefore, the IPS-type liquid crystal display device (hereinafter, referred to simply as an LCD) is mainly and increasingly used.
In fabrication of the LCD, in order to secure the cell gap 70, into which the liquid crystal 71 is put in a hermetically sealed manner, between the CF substrate 61 and TFT substrate 51, a columnar spacer (not shown) made up of an insulating material is disposed between the CF substrate 61 and the TFT substrate 51. Though the liquid crystal 71 is put in a hermetically sealed manner between the CF substrate 61 and the TFT substrate 51, the liquid crystal 71 expands or shrinks depending on a change in ambient temperatures. Therefore, the columnar spacer has to be disposed SO that a liquid crystal panel is formed in a manner that it is somewhat crushed at ordinary temperatures. Moreover, the columnar spacer has to be disposed so that the cell gap 70 is formed uniformly within faces being opposite to each other between both the CF substrate 61 and TFT substrate 51. However, there is a “trade-off” between these two needs. To satisfy these two needs simultaneously, a columnar area ratio CA defined as below has to be set within a range, as a precondition. The columnar area ratio CA is defined as follows:Columnar area ratio CA=(Px)·(Py)/(Lx)·(Ly)where Px denotes a horizontal length of the columnar spacer, Py denotes a longitudinal length of the columnar spacer, Lx denotes a horizontal length of each of the unit pixels including the unit pixel for the R color, unit pixel of the G color, and unit pixel for the B color, and Ly denotes a longitudinal length of each of the unit pixels including the R, G, and B color pixels. That is, the columnar area ratio CA is defined as a ratio of a cross sectional area of the columnar spacer to an area of each unit pixel.
The applicant of the present invention has already found that the above two needs can be approximately satisfied by disposing the columnar spacer so that the above columnar area ratio CA is set within a range of 0.05% to 0.15% (refer to Japanese Laid-open Patent Application No. 2001-117103, published on Apr. 27, 2001 after the filing date of Japanese Patent Application No. 2000-342163 of which the present application claims priority)
On the other hand, to avoid reduction in an effective area of the liquid crystal panel, it is desirous that the number of the columnar spacers is small and its sizes, that is, its horizontal length Px and its longitudinal length Py are small. Size of the columnar spacer is determined depending on fabrication accuracy of photolithography. If the size is too small, it is unstable in terms of strength. Therefore, both the horizontal length Px and the longitudinal length Py are set at approximately 8 μm or more. Moreover, the columnar spacer is disposed on a gate electrode (gate bus line) of the TFT which can provide a uniformly wide and flat place on the TFT substrate 51 and the horizontal length Px and longitudinal length Py have to be set so that they are smaller than a width P(G) (approximately 13 μm) of the gate electrode (not shown). Furthermore, when the horizontal length Px and longitudinal length Py of the columnar spacer have to be determined, it is necessary to take into consideration a shift “n” (approximately 3 μm or more) in superposition of the TFT substrate 51 on the CF substrate 61.
Also, when the columnar spacer is disposed, consideration has to be given to a column density. The column density is defined as a ratio of the number of columnar spacers to one set of pixels 75 made up of three kinds of unit pixels for R, G, and B colors. For example, as shown in FIG. 13A, when one columnar spacer 76 is disposed at any one (for example, the unit pixel for G color) of the unit pixels contained in one set of pixels 75, the column density is defined as “1/1”. Moreover, as shown in FIG. 13B, when one columnar spacer 76 is arranged in two sets of the pixel 75, the column density is defined as “1/2”. As shown in FIG. 13C, when one columnar spacer 76 is arranged in three sets of the pixel 75, the column density is defined as “1/3”. This means that, when the configuration shown in FIG. 13A having the column density being “1/1” is considered as a standard configuration, the columnar spacer 76 in the configuration shown in FIG. 13B having the column density being “1/2” is thinned out, that is, the number of the columnar spacers 76 is reduced by a half and in the configuration shown in FIG. 13C having the column density being “1/3”, the number of the column spacers 76 is reduced by one third.
If the horizontal length Lx and longitudinal length Ly of each of the unit pixels for the R, G, and B colors, and the horizontal length Px and longitudinal length Py of the columnar spacer 76 in FIGS. 13A to 13C are set at values as shown in a lower part of FIG. 13A, the columnar area ratio CA can be calculated by using the expression shown above and the following values can be obtained.    {circle around (1)} In the case of the column density being “1/1”->Columnar area ratio≈0.19%    {circle around (2)} In the case of the column density being “1/2”->Columnar area ratio≈0.095%    {circle around (3)} In the case of the column density being “1/3”->Columnar area ratio≈0.063%
Therefore, when compared with the columnar area ratio CA used as the precondition described above, the values obtained in the above cases of {circle around (2)} and {circle around (3)} are within the optimum range (0.05% to 0.15%). However, since a value being close to a mean value of the optimum range is preferable in actual operations, it is desirous that the columnar spacer 76 is disposed so that the configuration having the column density being “1/2” (the case of {circle around (2)}) boxed by a frame 77 shown in FIG. 13B can be provided.
FIGS. 14A, 14B, and 14C are also diagrams explaining the column density of the columnar spacer 76 in which the horizontal length Lx and longitudinal length Ly of each of the unit pixels remain the same as those in FIGS. 13A to 13C and the horizontal length of Px and longitudinal length Py of the columnar spacer 76 are different from those in FIGS. 13A to 13C.
The columnar area ratio CA of each of the cases is as follows.    {circle around (1)} In the case of the column density being “1/1”->Columnar area ratio≈0.285%    {circle around (2)} In the case of the column density being “1/2”->Columnar area ratio≈0.142%    {circle around (3)} In the case of the column density being “1/3”->Columnar area ratio≈0.094%
Therefore, in the example, values obtained in the cases {circle around (2)} and {circle around (3)} are within the optimum range of the columnar area ratio CA used as the precondition. However, for the same reason as above, it is desirous that the columnar spacer 76 is disposed so that the configuration having the column density being “1/3” (the case of {circle around (3)}) boxed by a frame 77 shown in FIG. 14C can be provided.
As described above, by changing the horizontal length Px and longitudinal length Py of the columnar spacer 76, the column density that can satisfy the precondition is changeable. By changing the horizontal length Lx and longitudinal length Ly of each of the unit pixels, the column density that can satisfy the precondition is also changeable. However, when the horizontal length Px and longitudinal length Py of the columnar spacer 76 are to be changed, the change must be within the range of constraints described above.
FIG. 15 is a top view schematically showing configurations of the conventional LCD in which the columnar spacer 76 is disposed so that the columnar area ratio CA is set within the optimum range used as the precondition and the column density becomes “1/2”. As shown in FIG. 15, the conventional LCD is so configured that two columnar spacers 76 are arranged in four sets of the pixel 75 indicated by broken lines (two sets along a row direction and two sets along a column direction), that is, one columnar spacer 76 is arranged in two sets of the pixel 75. Here, the columnar spacer 76 is disposed in, for example, the same unit pixels for G color and also in every other set of pixels 75 in a staggered manner. Thus, by disposing the columnar spacer 76 so that the columnar area ratio CA can satisfy the precondition and the column density becomes “1/2”, an elastic compositional deformation of the columnar spacer 76 is well balanced, which enables the columnar spacer 76 to be adaptable to changes in a thickness of the cell gap 70 caused by ambient temperatures.
However, the conventional LCD has a problem. That is, in the conventional LCD, since the columnar spacer 76 is arranged in the unit pixels for a same color in every other set of pixels 75, when the liquid crystal 71 is driven, all the columnar spacers 76 bear signal charges being the same in polarity, which causes disturbance in a transverse electric field caused by the signal charge being the same in polarity.
This problem in the conventional LCD will be explained below by referring to FIG. 16. FIG. 16 is a diagram explaining a method for driving the conventional LCD. The liquid crystal making up the LCD has a property that it is crushed by continued application of a voltage being the same in polarity (positive or negative). To avoid this, the liquid crystal 71 (not shown in FIG. 16) in the conventional LCD is driven by a dot reverse driving method in which a voltage being opposite in polarity is always applied alternately to the same unit pixel. Therefore, as shown in FIG. 16, a positive signal charge or a negative signal charge is written alternately in every unit pixel being arranged along the row direction X and a negative signal charge or a positive signal charge is written alternately in every unit pixel being arranged along the column direction Y so that each of the unit pixels adjacent to each other bears the signal charge being opposite in polarity. The reason why the positive or negative voltage is applied alternately to the unit pixels being adjacent to each other is to prevent flicker in displaying.
If the columnar spacer 76 is disposed in the unit pixels for the same color, for example, the G and G color pixels, for every other set of pixels 75, as in the case of the conventional LCD shown in FIG. 15, since the signal charge being the same in polarity is always written in every other unit having the columnar spacer 76 being arranged in the column direction Y, the signal charge causes the columnar spacer 76 to be electrically charged. That is, the columnar spacer 76 disposed in the unit pixel “G” bears the electrical charge being the same in polarity (positive or negative). As a result, the traverse electrical field is disturbed by influence of the signal charge being the same in polarity. In FIG. 16, there is shown a range 78 in which the traverse field is disturbed and shows that the range extends to an entire region in the column direction Y in which the columnar spacer 76 is arranged. On the other hand, since there is no object to be charged in the unit pixel having no columnar spacer 76, no disturbance in the traverse field occurs.
In the case of the IPS-type LCD in particular, since a method is employed to decrease specific resistance of the liquid crystal 71 in order to inhibit image retention, when the columnar spacer 76 is electrically charged, the electric charge in the liquid crystal panel gathers and the local specific resistance of the liquid crystal 71 is changed. Therefore, in the conventional LCD whose liquid crystal 71 is driven by the dot reverse driving method, if the columnar spacer 76 is disposed so that it is charged only when the signal charge having one polarity is applied, only electric line of force of the pixel voltage having one polarity enters the columnar spacer 76 and, as a result, the specific resistance of the liquid crystal 71 existing near the columnar spacer 76 is changed, which causes a failure in displaying such as flicker even when the liquid crystal 71 of the LCD is driven by the dot reverse driving method.